Link building is the part of search engine optimization and it is used for increasing the number and quality of inbound link to a webpage.
"Link building is the process of acquiring hyperlinks from other websites to your own. A hyperlink (usually just called a link) is a way for users to navigate between pages on the internet. Search engines use links to crawl the web; they will crawl the links between the individual pages on your website, and they will crawl the links between entire websites." You may read the rest here.